1. Field of the Invention
The invention relates to a plastic container of a high resistance to chemical attack and to a method for producing a the plastic container.
2. Prior Art
The fundamental problem exists of furnishing an inexpensive, easy-to-use plastic container with high chemical resistance and a highly protective effect against oxygen for oxygen-vulnerable products to be contained, such as permanent-waving preparations based on ammonium thioglycolate, hair dyes with ammonia or monoethanolamine, temporary hair colorings with direct-drawing pigments based on emulsions, cosmetic hair products with alcohol in a proportion of greater than 30%, and cosmetic hair products with a low pH of less than 4.
It is true that (extrusion blow molded) hollow bodies of PVC with high chemical resistance are known, but they are deficient in their ecological properties and limitations in certain recyling processes. In addition, (extrusion blow molded) hollow bodies of modified polyethylene terephthalate (PET) are known. From glycol modification, for instance, these modified polyesters have a higher melt viscosity, to lend them adequate tube stability for the extrusion blow molding process. When this material is used for the above products to be contained, however, a chemical attack takes place, which can lead to embrittlement, clouding, softening, and stress cracks.
Using pure polyethylene terephthalate (PET) is possible only in those blow molding processes in which the preform can be made by injection molding, since the melt viscosity is too low for processing by extrusion blow molding. Although there is indeed chemical resistance, nevertheless because of the injection blow molding process, developing and realizing a hollow body of this kind entails major investment for tools, long development lead times, major effort if the shape of weight of the hollow bodies is to be changed, and high production runs of at least 1 million pieces per year, because of the commercial requirements of the process.
In contrast to PET, using polyethylene naphthalate (PEN) also provides high chemical resistance against hair dyes, but even PEN has to be processed by injection blow molding, and thus has all the disadvantages named above for PET.
The use of multilayer hollow bodies based on polyolefins, such as PE/PA, is also possible, but with the following disadvantages: swelling of the PE, pigment creepage into the PE, problems with the welding seams in the hollow body, and the fact that adequate transparency if needed cannot be attained. The process is also expensive, because of the three layers and the adhesion promoter that has to be used.
From German Patent Disclosure DE 44 15 549 A1, an extrusion blow molded three-layer hollow body is known whose layers (from the inside out) comprise PETG, adhesion promoter and polyolefin. Only a water vapor barrier and a creepage barrier are achieved here. The disadvantages of the three-layer process, as described above, still pertain. Transparent containers cannot be produced. Nor is adequate chemical resistance achieved.
In addition, extrusion blow molded two-layer hollow bodies are known, which comprise two materials with affinity for one another, selected from the same group of materials, and which therefore require no adhesion promoter. These hollow bodies are used for instance when making a glossy layer on polyolefin bodies. In that case the main wall is blown from a nonglossy polyolefin with high melt viscosity, and a glossy layer of the same or similar material with low melt viscosity is then applied by the coextrusion blow molding process.